Transfer system for suboceanic oil production

ABSTRACT

A transfer system for offshore petroleum production has a vertically movable riser extending from a collection tank on the ocean floor to a storage on the ocean surface. The riser is releasably attachable by a pivotal connection to the tanker during flowing operation and disconnectable during storms or otherwise violent sea states. In the disconnected mode, the riser remains submerged under the ocean surface to avoid excessive structural loading. The riser is articulated and moored by a system of weights and floats to maintain tension within acceptable stress limits throughout a wide range of changes in vertical and some horizontal movement of the tanker.

United States Patent TRANSFER SYSTEM FOR SUBOCEANIC 01L PRODUCTION 1Claim, 6 Dnwing Figs.

u.s. c1 141/1,

1111. c1 B65b 1 04,

I B65b 3/04 Field of Search 141/346,

[56] References Cited UNITED STATES PATENTS 3, l 00,006 8/ l 963 Sheetset a]. 141/] Primary ExaminerH0uston S. Bell, .lr. AtlorneysL. LeeHumphries, Harold H. Card, Jr. and

Charles F. Dischler ABSTRACT: A transfer system for offshore petroleumproduction has a vertically movable riser extending from a collectiontank on the ocean floor to a storage on the ocean surface. The riser isreleasably attachable by a pivotal connection to the tanker duringflowing operation and disconnectable during storms or otherwise violentsea states. In the disconnected mode, the riser remains submerged underthe ocean surface to avoid excessive structural loading. The riser isarticulated and moored by a system of weights and floats to maintaintension within acceptable stress limits throughout a wide range ofchanges in vertical and some horizontal movement of the tanker.

PATENTED JUL2 7 m:

SHEET 1 OF 2 Y LA XAQQAX TRANSFER SYSTEM FOR SUBOCEANIC OIL PRODUCTIONBACKGROUND OF THE INVENTION.

In offshore petroleum production, offloading of crude oil collected fromvarious points on the ocean floor to a storage tanker or other floatingfacility is necessary. One method in current widespread use involvesattachment of a riser to a floating swivel buoy which in turn forms aconnection point for storage tankers. During storms or severe seastates, the tanker can be disconnected from the buoy, but the buoy canthereafter tear loose from the riser resulting in damage and possiblecomplete loss of the riser.

Another severe problem in the use of risers is associated with theirparticular sensitivity to structural overstress, especially in waters ofgreat depth or in offshore locations characterized by wide variation oftide levels. Thus, when a tanker heaves upward and downward due tosurface wave conditions, the riser connected to such tanker cannotusually respond to such movement at a sufficient rate to accommodate theresulting loads, although risers are generally stronger under tensileloads than any other type loading. During the downward displacement ofthe tanker, compression forces are applied to the riser which tend tomove the same downwardly. The huge length, inertia and relativeinflexibility of the riser as well as tremendous hydrostatic pressuresseverely retard its downward movement through the water, wherebycompression loads identified the mentioned operating conditions involvegreater risk of damage than any other type of load or stress imposedthereon. Distortion of the riser such as identified with column .bendingin beams or the like or any distortion resulting in curvature of theriser cannot be accommodated in view of its mass and axial rigidity.

SUMMARY OF THE INVENTION The invention contemplates a riser consistingessentially of two separate runs or links I2 and 14 for communicatingbetween a collection or storage tank 16 on the ocean floor and a tankerI8 on the ocean surface. Links 12 and I4 are both substantially straightand are joined to each other by a pivotal connection 20. Divergingmooring lines connect joint with two floats 26, 27 and two anchors 28,29, respectively, as shown in FIG. 2, to maintain link I2 substantiallyunder tension throughout a relatively wide range of vertical andhorizontal displacements of joint 20 as suggested in FIG. I. Weightedmass is secured to joint 20 to apply downward force to vertical link 14,while upward force is applied to the same riser link by buoyant mass orfloat 32, thus maintaining this portion of the riser under continuoustension during the mentioned range of movement. The upper end of riser14 is releasably connected to tanker 18 in the offloading operation, andwhen disconnected therefrom remains submerged under the ocean surfaceand therefore less exposed to violent forces associated with stormy seasor collision with surface vessels.

Connection of riser 14 with tanker 18 occurs through a mooring swivel 76on the storage tanker, at the top of which suitable locking means andconduit flow connections are situated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a side elevationalschematic view of the riser and mooring system contemplated in thiscase,

FIG. 2 shows a top plan view ofthe system shown in FIG. 1,

FIG. 3 shows a detailed view of the upper end of the riser shown in FIG.I, but in the disconnected mode,

FIG. 4 shows a fragmentary view, partly in cross section, of a detailfrom the structure shown in FIG. 3,

FIG. 5 shows a transverse view, partly in cross section, through astorage tanker connected to the riser shown in FIGS. 1 and 2, and

FIG. 6 is a top plan view of the connection system shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT From FIG. I, it may be seen thatthe invention in this case contemplates and includes a collection orstorage container 16 for oil or gas accumulated from one or moresuboceanic well drillings (not shown) supplied to tank 16 by one or moreconduits as suggested by conduit 10. A transfer riser consisting of twosections or links 12 and I4 communicates the internal area of tank 16with a storage tanker or other floating facility 18 on the oceansurface. Riser link 12 is operatively joined to tank 16 through pivotaljoint 36 which may take any convenient form known to the prior art,including joint 17 shown in US. Pat. No. 3,236,266 issued Feb. 22, I966and permitting rotational movement of link 12 in a substantiallyvertical plane perpendicular to horizontal axis 38 through joint 36 asshown in FIG. 2. Link 12 of the riser is connected to link 14 through asecond pivotal joint 20 which is movable in an arc defined by a centerof rotation coinciding with axis 38. The arcuate path of movementinvolves both vertical and lateral displacement ofjoint 20 as suggestedby the dash lines in FIG. I. Restraining means are provided to limit thetransition rate at which movement ofjoint 20 may occur, the stated meansincluding relatively heavy mass or weight 30. Link 12 is renderedneutrally buoyant by suitable hydrostatic balance means such as bysecuring any required number of floats spaced along the length of link12 as may be necessary to compensate for the weight and massdistribution of riser link 12, which will naturally include dueconsideration of the ocean depth and displacement volume of link 12 inany particular installation.

Due to the particular sensitivity of riser links 12 and 14 to damage bycompressive loads applied axially thereto, means are provided in thiscase to maintain each of the separate links under substantially axialtension. The stated means includes one or more mooring line connectionsbetween joint 20 and one or more anchors or dead weights. Preferably, aminimum of two separate mooring line connections are used to applytensile loading of link I2. Thus, referring to FIG. 2, lines 22 and 23are attached to joint 20 and connect the same with buoyant masses orfloats 26 and 27, respectively. Mooring lines 24 and 25 each connectfloats 26 and 27 with separate anchor or deadweight means 28 and 29,respectively. Anchor means 28 and 29 are positioned on the ocean floorequidistantly from the longitudinal axis of riser link 12 so thatlateral movements of link 12 either in a clockwise or counterclockwisedirection about joint 36 as shown in FIG. 2 will be resisted bysubstantially equal reaction loads in lines 22 and 23. In achieving theforegoing action affects, the included angle 40 between lines 22 and 23should preferably be within a range from 20 to the higher range limitbeing preferred. Tensioning of riser 12 is achieved by both of theseparate branches in the same manner and may be illustrated by the oneshown in FIG. I comprising elements 22, 24, 26, and 28. Due to buoyancyof float 26, it will be understood that tension is maintained in lines22 and 24, for example, connected to joint 20 and anchor 28,respectively, and that a force component due to the started tension inline 22 pulls joint 20 generally toward the right in the view shown byFIG. 1. Since joint 20 is structurally connected to link 12, the pullingforce thus applied to the joint results in tensile loading of link 12 asthe link is unable to move axially due to the restraint offered by itsconnection with joint 36 and stationary tank 16. The vertical positionofjoint 20, to which downward pulling force is continuously maintainedby attachment of weight 30 thereto, is dependent upon the location ofthe upper end of riser link I4, which in turn depends upon the buoyancycharacteristics of riser buoy 32 in the disconnected mode or theposition of tanker 18 when the riser is connected thereto.

In the connected mode suggested by FIG. 1, it will be understood thatocean surface 42 will cause variations in the vertieal position oftanker 18, either through wave action or tides, and also through loadingof the tanker as its hold is filled. Changes in the vertical position oftanker 18 with respect to the ocean floor 44 during oil transferoperation of the riser will result in displacement ofjoint 20 along thearcuate path within which the joint is movable. If, for example, tankerl8 heaves upwardly from the initial position shown in FIG. I, upwardforce transmitted through riser 14 will pulljoint 20 upwardly, and suchupward movement will necessarily result in lateral movement of riser l4and joint 20 due to the restraint offered by riser portion 12 throughits connection with the joint. The stated displacement due to upwardheaving of tanker 18 is suggested by dashed line 14 denoting thedisplaced position of riser [4 toward the left in FIG. 1. In the statedcondition of displacement, float 26 will be forced in a generallydownward direction as lines 22 and 24 attached thereto are pulled intocloser linear alignment by movement ofjoint 20 with respect tostationary anchor means 28. The displaced position of float 26 due tomovement of riser l4 and joint 20 toward the left in FIG. I is suggestedby reference numeral 26. In the displaced condition thus represented bynumerals I4 and 26 in FIG. I, it will be understood that tensile forceis continuously maintained axially through both riser links 12 and 14due to the horizontal force component of the tension in lines 22 and 24and through the continuous downward pull applied tojoint 20 by mass 30.

Downward heaving of tanker 18, to which riser link 14 is connected atits upper end, would result in corresponding downward movement of joint20 due to the continuous force applied to joint 20 by mass 30. Thestated downward movement ofjoint 20 would be accompanied by lateralmovement of riser 14 toward the position designated by dashed line 14"to the right as shown in FIG. 1 due to the continuous rightward forceapplied to joint 20 by float 26 and transmitted through 22. Downwardmovement ofjoint 20 in the foregoing manner would cause correspondingdownward movement of float 26 to the position suggested by referencenumeral 26" by force transmitted through line 22 extending between thejoint and the float. In the displaced condition thus denoted byreference numerals 14" and 26" in FIG. 1, it will be understood thatriser links 12 and 14 are both continuously maintained under tension dueto the forces respectively applied thereto by float 26 and mass 30,respectively. It may incidentally be noted that the position of thestructure shown in FIG. I and identifiable with reference numerals l4"and 26" is the one normally assumed by the riser and mooring system whenriser I4 is in the disconnected mode as discussed more fully below. Inthe foregoing disconnected position, mass 30 may descend so far as totouch ocean floor 44, which in many suboceanic locations may comprise asemisolid surface of mud rather than a smooth hard surface of rock orthe like. To prevent embedment of mass 30 by sinkage thereof into ahighly tenacious mud mass, a mud mat 46 may be provided in the locationsuggested by FIG. I for supporting mass 30 at its lowermost limit ofmovement.

Referring to FIG. 3, the upper end of riser 14 is shown in thedisconnected mode characterizing its condition when not in use fortransferring products from tank 16 into a storage facility or the likeon ocean surface 42. Hydrostatic balance means in the form of buoyantbody 32 comprises a hollow sphere through which riser l4 penetrates issecured or otherwise formed on the riser. The upper terminal end of theriser is provided with a blind hub or cap 48 which seals and closes offthe upper terminal end of riser l4. Cap 48, as shown more particularlyin FIG. 4, may include a sealing surface 50 adapted to make contact witha sealing member such as flexible ring 52 nested within the upperterminal edge portion of riser l4, and held firmly in contact therewithby suitable clamping means such as an overcenter toggle system as shownin the mentioned figure. The stated system may take any suitable formincluding those known to the prior art, and illustratively may comprisea projecting boss or lug 54 integrally formed on cap 48 and supporting apivotal link 56 having a pivotal lever 58 mounted on the lower endthereof. A projecting flange or curved lug 60 integrally formed orotherwise secured to riser 14 is adapted to receive a rounded portion oflever 58 in securely nesting and load transmitting relationship as shownin FIG. 4. Rotation of lever 58 mto or out of engagement with flange 60provides secure but easily disconnected holding means between the capand the riser. As further shown in FIGS. 3 and 4, cable connection meansare provided on cap 48 for securing visual marker buoy 62 thereto. Thestated mans illustratively include a hollow cleat 64 having cable 66joined thereto in the manner shown by FIG. 4. Riser buoy 32 has apredetermined amount of buoyancy which, in combination with the liftingforce of buoyant float 32 and the downward forces applied by riser I4and components attached thereto, result in buoy 32 hanging at apredetermined depth below ocean surface 42 a sufficient amount to avoidthe violent effects associated with stormy seas and the like.

As may he further seen from FIG. 4, the terminal end portion of riser I4is provided with an annular groove 70 defined by two verticallyspaced-apart annular flanges 72 and 74. Referring particularly to FIG.5, attachment means for connecting riser 14 to tanker 18 mayconveniently include pivotally mounted mooring swivel 76 supportedwithin a vertical passage 78 through the center of tanker l8. Suitablebearing support for mooring swivel 76 is provided as schematically shownby bearings 80 and 82, for example.

In operation, a line (not shown) may be passed through center passage 78oftanker l8 and secured by marker buoy 62 or cable 66 in order to pullthe sealed and submerged end of riser 14 up through mooring swivel 76for operative attachment as required during the transfer operation. Withthe riser and buoy 32 thus positioned as shown in FIG. 5, locking colletor annular flange means mounted on mooring swivel 76 are secured toriser 14 within groove 70 and locked in place by suitable meansincluding those known to the prior art. Blind cap 48 is then removed,exposing the open end of riser l4 and positioning the same substantiallyin the center of passage 78. A pivotally mounted section of pipe 84,with a suitable pipe swivel 99, appropriately supported on tanker 18such as by brackets 86 and having mating connection means on the distalend thereof as suggested by flanged end 88 in FIG. Sis rotated intomating connection with seal 52 at the terminal end of riser l4. Pivotallinks 90 secured at one end to a swiveling mooring structure by suchmeans as brackets 92 may be engaged to similar brackets 94 proximate theend 88 of pipe 84 and joined thereto by a pin and clevis connection orthe like to hold the pipe 84 in tightly sealed fluid transferringrelationship with riser 14. Alternatively, a toggle system such as shownin FIG. 4 for cap 48 may be used on flange 88 of pipe section 84 tosecure the same together. When thus connected, the contents of tank 16on ocean floor 44 may be transferred into storage tanks and the likeaboard tanker 18 through suitable conduits as suggested by pipe 96 inFIG. 6. Disconnection of riser 14 from tanker 18 is accomplished bysimply reversing the connection operation, and includes replacement ofcap 48 in the position shown by FIG. 4 and release of the riser togetherwith marker buoy 62 passage 78 into the submerged state.

We claim:

1. A method of transferring fluids from a collection point on the oceanfloor to a tanker ship on the ocean surface, comprising the steps of:

lowering a line through a center aperture through said ship,

securing said line to a marker buoy on said ocean surface attached to asubmerged transfer riser below said ocean surface,

lifting said riser through said aperture, and

connecting said riser to flow conduits on said tanker ship.

1. A method of transferring fluids from a collection point on the oceanfloor to a tanker ship on the ocean surface, comprising the steps of:lowering a line through a center aperture through said ship, securingsaid line to a marker buoy on said ocean surface attached to a submergedtransfer riser below said ocean surface, lifting said riser through saidaperture, and connecting said riser to flow conduits on said tankership.